drm/atomic: fix potential null ptr on plane enable

When a plane is being enabled, plane->crtc has not been set yet.  Use
plane->state->crtc.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_atomic_helper.c

@@ -61,7 +61,7 @@ drm_atomic_helper_plane_changed(struct drm_atomic_state *state,
 	struct drm_crtc_state *crtc_state;
 
 	if (plane->state->crtc) {
-		crtc_state = state->crtc_states[drm_crtc_index(plane->crtc)];
+		crtc_state = state->crtc_states[drm_crtc_index(plane->state->crtc)];
 
 		if (WARN_ON(!crtc_state))
 			return;